Increasing demand for convenient, reliable, and clean energy systems has drawn much attention to the development of fuel cells.
A fuel cell is an electrochemical device that produces electricity by controlling the flow of atoms and electrons during a reaction between a fuel and an oxidant to make use of the exchange of electrons associated with such reactions. The fuel is typically hydrogen, natural gas, coal gas, or other hydrocarbon-based fuel, and the oxidizer is typically air, oxygen, or other oxidizing composition.
A typical fuel cell generally contains a cathode, anode, electrolyte, and interconnect. The electricity-generating electrochemical reaction is carried out in a fuel cell in a controlled, stepwise manner. For example, the cathode causes oxygen to be reduced to oxygen ions. The solid electrolyte regulates the flow of molecules, acting as a barrier to all but oxygen ions. In addition, the anode catalytically extracts electrons from the fuel. And the interconnect transports the electrons from the anode to the cathode to complete the process and energy generated may be harnessed for various applications, such as powering an automobile.
Several families of fuel cells are known in the art. Although the basic components and function of each family is similar, the details of operation and the materials used in construction, however, are significantly varied. The present invention relates, generally, to fuel cells utilizing a solid electrolyte, i.e., SOFC.
A typical SOFC contains a solid, oxygen-ion conducting electrolyte separating a fuel electrode (anode) from an air electrode (cathode). During operation, electrons are released and the electrochemical potential (e.g., on the order of 1 volt for a single fuel cell) of the reaction draws the electrons through a circuit, preferably, an external circuit, where this electromotive force is utilized. Because the voltage/power output of a single fuel cell is relatively low, a typical unit targeted for commercial sale would include a plurality of fuel cells connected in series, parallel, or combinations thereof, through an electrical interconnect. This combination of fuel cells is known in the art as a stack.
Currently, zirconia is a generally the material of choice for the electrolyte. However, zirconia has a relatively low ionic conductivity, therefore it is often impregnated with yttria which introduces increased ionic conductivity (hereafter referred to as YSZ or yttria stabilized zirconia). The anode is typically made of a Ni/YSZ cermet and the cathode, a perovskite composition.
There are generally three types of SOFC: tubular, planar, and monolithic SOFC. Each of these fuel cells is similar in basic structure (anode, cathode, solid electrolyte, etc.), but differs substantially in geometry. Furthermore, each type of SOFC may further have a variety of different configurations. For example, a SOFC may be an electrolyte-supported type SOFC, where a dense electrolyte is used as supports and electrodes are applied on each side of the electrolyte (See, e.g., U.S. Pat. No. 5,273,837 to Aitken et at. and U.S. Pat. No. 6,428,920 to Bedding et al.). A SOFC may also be an air electrode-supported type SOFC, where the air electrode contains an inner porous impregnated-lanthanum manganite substrate subsequently coated with a gas-tight electrolyte layer, anode, and interconnect coatings (See, e.g., U.S. Pat. No. 5,108,850 to Carlson et at. and U.S. Pat. No. 5,989,634 to Isenberg). Additionally, a SOFC may be a fuel electrode-supported type SOFC, where the anode cermet acts as the support and the electrolyte layer underlying the air electrode is coated as a thin film (See, e.g., U.S. Pat. No. 5,998,056 to Divisek et al. and U.S. Pat. No. 6,228,521 to Kim et al.).
The output power of a SOFC is a function of the operating temperature, the area specific resistance (ASR) of the fuel cell, and the overall ionic conductivity of the electrolyte and the electrodes. The ASR and ionic conductivity are traditionally optimized by adjusting the starting compositions of the materials in the electrode and/or the electrolyte, and then further refined by adjustment of particle size and sintering conditions to give the desired morphology.
The area specific resistance (ASR) of the anode, for example, may be influenced by the Ni content of the electrode. For example, lower Ni content gives more stable electrode structures with better thermal expansion characteristics. However, to achieve good electrical conductivity and a low ASR, the Ni must form a coherent conducting “network” which generally requires a higher Ni content.
The ionic conductivity of the electrode also has, a major influence, on the power produced. The electrochemical reactions that drive the fuel cell are conventionally considered to occur at or around the three phase boundary (TPB), where the electrolyte meets the electrode and the electrode is in contact with the reactant gas. This area is generally considered to exist at the actual interface between the electrolyte and the electrode. If the ionic conductivity of the electrode could be substantially increased without detrimentally affecting the electrical properties, then substantial power increases may be obtained. This increased conduction of oxygen ions through the actual electrode may also reduce carbon deposition.
The operating temperature of the fuel cell may be controlled by limiting heat loss through the internal walls of the stack compartment and by transferring a fraction of the latent heat to the incoming fuel and air streams. During start-up, the stack may be heated via an external heat source such as electrical heating or via an internal heat source, such as combustion of a proportion of the available fuel. The ionic conductivity of the electrolyte is a function of temperature. Therefore, it is imperative that the desired operating temperature is reached as rapidly as the fuel cell and stack components will allow.
Operating temperature may be an important design parameter. High temperatures add complexity to the overall design, e.g., affecting the type of seals that may be employed. Operating temperature may also affect the efficiency of the fuel cell. SOFCs are generally operated at temperatures ranging from 800° C. to 1,100° C., which are required to overcome the high resistance of the electrolyte and polarization losses of the air electrode encountered at low temperatures. Recently developments that lead to lower temperature operation may be useful when the system operates on hydrogen, but are generally not practical when certain hydrocarbon based fuels are used as the fuel until more efficient low temperature reforming catalyst are developed.
One of the inherent advantages of the SOFC is its potential to operate on a variety of fuels, including hydrocarbon fuels. These fuels should either be reformed directly at the anode, or in-directly in a reforming unit within or near the stack. Reforming is traditionally accomplished at elevated temperatures, and thus integration within the stack produces a more efficient overall system design.
Direct reforming is preferred over in-direct or external reforming as benefits from coupling exothermic fuel cell reactions and endothermic reformer reactions increase efficiency and the integrated design greatly simplifies and reduces the size of the system. Unfortunately, the anode electrode known in the art is susceptible to carbon deposition when operating on hydrocarbon fuels.
SOFCs are being developed by numerous groups and hold great promise for commercial success due to their many potential benefits. Among these, the possibility of operating on a variety of fuels, efficient energy conversion, and low pollution are primary motivators. Multi-fuel capability is an important characteristic, considering that hydrogen is not widely available at present, nor is it expected to be easily accessible for many years.
However, successful commercialization demands a system that may be manufactured consistently, and that it is reliable, efficient, and requires simple or little maintenance. To date, several problems associated with SOFCs remain a barrier to its successful commercial use. For example, current art has not completely overcome efficiency losses due to poor ionic and electronic conductivity. Electrode structures which may produce ideal electrochemical characteristics are not always stable and may be susceptible to sintering. Whereas, electrodes that have a stable morphology often have a low surface area and may have a high ASR. Additionally, to develop a commercially viable SOFC, it is necessary to produce a fuel cell with consistent power density and stability in a variety of operating environments. To date, manufacturers have had a difficult time making reproducible fuel cells, sometimes scraping a majority of them due to poor performance. Issues that may arise include: inconsistent electrode thickness, non-homogenous mixing of materials, and non-uniform coatings, which contribute to variations in fuel cell performance. Temperature variants within the calcination furnace may also cause deviations in fuel cell power output. Several additional factors may affect the operating life of a SOFC, including carbon fouling and densification of the anode. Carbon fouling takes place when the fuel stream includes carbon (e.g., when a hydrocarbon fuel such as propane is used). Deposition may lead to loss of performance and eventually a catastrophic fuel cell failure. Furthermore, commercially available hydrocarbon fuels often contain other chemicals, such as sulfur, which may poison the fuel cell by blocking active sites. Densification, or sintering, may occur within the fuel cell during operation, and aggravates with increasing temperature. This is most significant on the anode side where a metal is generally incorporated into the design. In addition, for some applications, the time required to reach substantially full power output is a significant concern. For example, it is extremely important for the automotive industry, where “instant-on” is a strict requirement for commercial success. For SOFCs, the start up time is dictated by the rate of temperature increase, as the fuel cell output is controlled by the temperature of the fuel cell. Systems using battery and super capacitor support have been proposed, but the volume occupied by such a system that generates an equivalent power is prohibitive for the applications. It is therefore desirable to have processing technology and a fuel cell that increases both electrons and ionic conduction while increasing resistance to carbon formation and sulfur poisoning.
Several attempts have been made to-overcome the aforementioned problems, such as conductivity losses, and resistance to carbon formation and sulfur poisoning in SOFCs.
One approach to increase ionic conduction is through the use of non-zirconia based electrolytes. For example, U.S. Pat. Nos. 4,851,303 and 5,134,042 disclose a variety of non-zirconium solid electrolytes, having a polycrystal or single crystal structure, such as lanthanum strontium lithium fluoride, calcium uranium, SrC12KCI, and others. The use of non-zirconia based electrolytes introduces significant challenges to the design of the integrated SOFC system and is not widely accepted, as witnessed by the relatively few efforts on non-zirconia systems, as compared to zirconia based electrolyte development. Thermal expansion matching of the electrodes and interconnect is one area that is complicated by the use on non-zirconia electrolytes. Furthermore, some non-zirconia electrolytes; such as some ceria-based electrolytes, exhibit electronic conduction in fuel atmospheres, resulting in excessive fuel consumption.
The use of a thin film YSZ electrolyte, such as that disclosed in U.S. Pat. Nos. 5,753,385 and 6,007,683, is another approach to reduce ionic losses. Vapor deposition techniques are employed to produce the thin films in both patents. U.S. Pat. No. 6,548,424 discusses an alternative process for production of thin film YSZ electrolytes, comprised of atomic layer deposition, wherein alternating vapor-phase pulses of constituent materials are fed into a reaction space and contacted with a substrate. While thin films are successful in reducing the resistance of the electrolyte by limiting the path length, the reliability of the fuel cell is compromised, as the structural integrity is significantly impaired. Gas leakage through the thin sections is another consideration, as it may result in degraded efficiency. In addition, manufacturing cost may be considerably higher with vapor deposition techniques, as compared to the powder processing approaches that are typically employed.
U.S. Pat. No. 5,993,989 discusses an interfacial layer of terbia-stabilized zirconia between the air electrode and the electrolyte that may be employed to reduces losses and operate over a wide range of temperatures. By incorporating the layer, some control of interaction between the air electrode and electrolyte and a reduction of polarization loss may be achieved. In U.S. Pat. No. 6,207,311, a Smaydia-stabilized zirconia electrolyte having high electrical conductivity is discussed, where the electrolyte material may be a very thin layer. Although the approaches discussed in the above patents may be used to reduce losses, the addition of an intermediate layer adds complexity, and therefore cost, to the manufacturing process. In addition, thin electrolytes are fragile and may result in an unreliable fuel cell.
Commercially viable manufacturing of SOFCs requires a process that is capable of producing a large quantity of fuel cells with consistent power density and stability in a variety of operating environments. Such a process is difficult to achieve with existing techniques. Furthermore, currently, once a fuel cell is sintered, very little if anything may be done with the fuel cell if its performance was not up to that of the others in the batch.
Accordingly, there remains a need in the art for a robust SOFC that may be manufactured in a reliable and consistent manner, and undergoes high efficiency operations and has an extended operational life. There also is a need in the art for a low cost process to produce such an SOFC, in such a way that it may be applied to a wide variety of electrolyte, anode, and cathode materials, and that is easy to, incorporate into existing manufacturing processes for a wide variety of SOFCs.